1. Field of the Invention
The present invention relates to a focus control device of an optical head in an optical recording and reproducing (playback) apparatus which performs recording/reproducing optically and, more particularly, to correction of a focus-error signal in a focus servo.
2. Description of the Related Art
As a focus detecting device for use in an optical recording and reproducing apparatus which performs recording/reproducing optically with respect to an optical recording medium, the applicant of the present application has proposed a device in Japanese Patent Laid-Open No. HEI 1-191340/1989, in which light from a light source is projected onto an optical recording medium through a collimator lens, a beam splitter and an objective lens under a defocus condition, and reflected light is received by an optical detector through an objective lens, a beam splitter and an imaging lens. A focus condition of the objective lens with respect to the optical recording medium is detected on the basis of illuminance distribution on the optical recording medium. Further, as a structural arrangement which improves the aforesaid device, a focus detecting device has been proposed in Japanese Patent Laid-Open No. HEI 3-80435/1991 in which an optical element having spherical aberration in an optical path from the light source provided, and a change in illuminance distribution in the form of a medium surface on the basis of spherical aberration on the optical element is detected to detect a focus condition of the objective lens with respect to the optical recording medium.
A structural arrangement of an optical system illustrated in the focus detecting device disclosed in Japanese Patent Laid-Open No. HEI 3-80435/1991 is shown in FIG. 1 of the attached drawings. An optical system is arranged such that light from a light source 1 is projected onto a recording medium 6 in the form of a card, through a sapphire sphere 2 having spherical aberration, a collimator lens 3, a beam splitter 4 and an objective lens 5, and reflected light from the recording medium 6 is received by a light detector 8 through the objective lens 5, the beam splitter 4 and an imaging lens 7. Furthermore, an actuator for driving the objective lens 5 is connected to the objective lens 5. The actuator is provided with a tracking coil 9 and a focusing coil 10. Thus, the objective lens 5 is driven in a tracking direction and in a focusing direction on the basis of a tracking error signal and a focus error signal.
As shown in FIG. 2, the optical detector 8 is used which is arranged by a plurality of light receiving cells. Each pair of four pairs of cells C1 and C2, C3 and C4, D1 and D2, and D3 and D4 for generating a tracking error signal and a focus error signal are so provided as to be spaced from each other in a track widthwise direction so as to be opposed against each other and are so arranged as to receive an image 11 of an edge portion of a guide pattern of the recording medium 6. Moreover, four pairs of clock generating cells B1 and B2 are arranged between the cells D1 and D2, and D3 and D4 so as to be spaced from each other in the track direction. Further, sixteen (16) data lead cells 13 are arranged in the track widthwise direction of a central portion so as to correspond to a data recording position of the recording medium 6.
The cells C1.about.C4 and D1.about.D4 for generating the focus and tracking error signal are connected to the focus error signal generating circuit 14 illustrated in FIG. 3 so that a focus error signal is generated on the basis of outputs from the cells C1.about.C4 and D1.about.D4. In this connection, although not shown, the cells C1.about.C4 and D1.about.D4 are connected also to the tracking-error-signal generating circuit so that a tracking error signal is generated. However, the detailed description thereof will be omitted. The cells C1.about.C4 are connected to a negative input end of a differential amplifier 15, respectively, through resistors R1. On the other hand, the cells D1.about.D4 are connected to a positive input end of the differential amplifier 15, respectively, through resistors R2. Moreover, the middle-point voltage is supplied to the positive input end of the differential amplifier 15 through a resistor Rg.
A sum signal (.SIGMA.C) of the outputs from the respective cells C1.about.C4 and a sum signal (.SIGMA.D) of the outputs from the respective cells D1.about.D4 form signals which vary depending upon a distance from a medium surface of the objective lens as shown in FIG. 4. Thus, there can be provided a focus error signal FE which becomes zero at a control target position if the respective sum signals .SIGMA.C and .SIGMA.D are amplified by gains different from each other and are subtracted. That is, a focus condition of the objective lens can be detected by the focus error signal FE. Here, the resistor Rg is a resistor which decides a gain with respect to a sum signal of the outputs from the cells C1.about.C4 and D1.about.D4. Assuming that R1=R2=20 k.OMEGA. and R3=40 k.OMEGA. and that a ratlo between .SIGMA.C and .SIGMA.D at the control target position is 1:1.5, then Rg becomes 0.86 k.OMEGA..
The focus error signal FE is supplied to the focusing coil 10 of the actuator through a driving step, and the objective lens 5 is driven, whereby the objective lens 5 is controlled so as to become a defocus condition which is a target with respect to the recording medium 6.
The aforementioned conventional focus detecting device, however, has a problem in that, as will be seen from FIG. 4, the focus error signal FE crosses a zero point twice. Specifically, there is a point where a value of the focus error signal FE becomes zero (secondary zero cross point) at a position where the objective lens 5 is largely spaced away from the recording medium 6. Accordingly, if the distance between the objective lens 5 and the recording medium 6 crosses the secondary zero cross point and is largely spaced further over the secondary zero cross point by refuse on the recording medium 6, defects, disturbance and the like, the following deficiencies occur. That is, there occurs a focus error signal having polarity which further spaces the objective lens 5 and the recording medium 6 away from each other. It is impossible to maintain the objective lens at a control target position.